Isolated sinusal dysfunctions, or those within the scope of the atrial illness, raise a question as to the choice of the optimal pacing mode. A single-chamber pacemaker for atrial pacing in the AAI operating mode appears most appropriate However, when the sinus deficiency extends to the bundle of His, paroxystic atrio-ventricular block may appear intermittently or completely. When it does appear, there is no atrio-ventricular conduction or spontaneous depolarization of the ventricle (the latter is also referred to as "ventricular detection"). In this situation, as long as the block exists, there is a need to provide pacing of the ventricle.
Therefore, as a safety measure, it has been the practice in such circumstances to implant a dual-chamber pacemaker which operates in the DDD mode to pace systematically the atrium and the ventricle. However, outside of a period of crisis, i.e., when there is no atrio-ventricular block, ventricle pacing is not necessary. A problem with ventricle pacing at such time is that it can cause competition between the paced and spontaneous depolarizations, and pacing consumes added energy for such unneeded pulses which shortens the useful life of an implanted, battery-operated pacemaker.
There is thus a continuing need for cardiac pacemakers having improved operating characteristics for pacing hearts that undergo temporary periods of crisis. There is a further need for more efficient and more adaptive pacemakers that change the pacing operating mode in response to physiological changes of the patient.